Method of coating conduits



June 18, 1968 F. E. MONULTY ETAL 3,389,009

METHOD OF COATING CONDUITS Filed July 22, 1964 INVENTOR. FRANK E. MCNULTY ROBERT M. NEE

United States Patent 3,389,009 METHOD OF COATING CONDUITS Frank E. McNulty and Robert M. Nee, Tulsa, Okla., as-

signors to Nee & McNulty, Inc., a corporation of Oklahoma Filed July 22, 1964, Ser. No. 384,445 4 Claims. (Cl. 117-18) ABSTRAEIIT 0F THE DISfiLGSURE Pipe or similarly shaped objects can be coated by means of a continuous process involving placement of a substantially uniform layer of coating material particles onto an endless or continuous carrier belt which moves at right angles to the longitudinal axis of the pipe being coated. The pipe is heated to a temperature usually from about 400 F. to about 600 F. and the particles of coating material are fused to the metal surface. Simultaneously on contact of the coating material with the hot surface of the pipe, the carrier belt separates from the applied coating, cooled, and a fresh charge of coating material laid on the carrier belt for a subsequent application to the surface of the heated pipe. This results in a pipe section with a protective coat of uniform thickness.

This invention relates to a novel method for applying finely powdered resin particles to a pipe or conduit to form a protective layer or coating. More particularly, it is concerned with a method of applying heat reactive epoxy resins and equivalent materials to a conduit without the disadvantages characteristic of the fluid bed and cloud chamber techniques currently used in the application of such resins.

Powdered resins have been employed as protective coatings for the inside and outside of drill pipe, oil well casing, tubing and other forms of conduit as a protective coating. These materials have been applied by heating the pipe to a temperature usually from about 400 F. to about 600 F. and fusing the coating to the metal surface. This has been done by passing the heated pipe through a cloud chamber where there is a high density of the resin solids suspended in an air stream, or by the fluid bed coating technique. In fluid bed operations, the individual pipe joint is placed in a bed of suspended, fluidized resin particles. In either the cloud chamber method or the fluidized bed procedure, the thickness of the applied coating depends on the pipe temperature and also on the characteristics of the particular resin.

In the cloud chamber method, which has been used widely for coating oil and gas line pipe to prevent external corrosion, the resulting coatings can have considerable variation in thickness. As the pipe is passed horizontally through the chamber, the thickness on top may be of the order of 15 mils and on the bottom about 9 mils. For example, if the quantity per cubic foot of powder injected into the air stream going to the cloud chamber is varied mechanically, the density of the powder in the chamber in relation to the air can cause a variation in the coating thickness, assuming the pipe is passing through the chamber at a uniform speed. Thickness variations of 50% or more are often obtained in cloud chamber coating operations, which is a waste of material, particularly where minimum thickness of 10 mils is desired. Experience has shown it is very difficult to hold accurate thickness tolerances in applying powdered resins by the cloud chamber method. While the fluid bed method of "ice coating is better in so far as concerns thickness control, it has a disadvantage in that it is not as adaptable to high speed operation and automation in the handling of the conduit as is true in the case of the cloud chamber type of operation.

It is customary in cloud chamber application to pass the pipe through laterally without the pipe being rotated. A cloud chamber that is required for large diameter pipe, therefore, is diflicult to design and for this reason no commercial applications thereof have existed for applying coating to pipe in excess of 8 inches in diameter. In this method also it is necessary to leave the ends of the cloud chamber open to allow for some sagging of the pipe, as the pipe cannot be suspended from the time it enters the cloud chamber until the coating has firmed enough to be quenched with water. Therefore, in blowing air into the chamber a certain amount of the entrained powdered resin will escape from the open ends of the chamber which wastes the resin and also presents a ventilation problem as the resins, generally speaking, are toxic.

Prior to placing the coating on the pipe by the cloud chamber or fluid bed methods, the surface to be coated is blasted with steel shot or sand in order to remove loose scale and other materials that might interfere with the formation of a permanent bond. The resulting cleaned surface is covered with peaks and indentations. Some of these peaks may be as much as 7 to 8 mils in height from a valley. If a thin spot in the coating, say less than 7 or 8 mils in thickness, coincides with a peak, then the top of the peak protrudes or is flush with the surface of the resulting coating. In either case a conductive system is created for electrolysis currents which, of course, can cause eventual failure of the pipe.

Accordingly, it is an object of our invention to provide a means for applying a powdered resin to a pipe without the use of either a fluid bed or a cloud chamber. It is another object of our invention to provide a means of applying a finely divided resin to produce a coating of more uniform thickness to a pipe or conduit. It is a further object of our invention to provide'a method for simultaneously placing a resin coating and a plastic or similar wrapping material on the conduit to be protected. It is still another object of our invention to prevent electrolytic corrosion of a pipe coated by the application of resin powders to heated pipe in which portions of metal remain exposed owing to the unusually rough surface of the pipe.

The accompanying drawing illustrates one type of apparatus suitable for applying finely divided resin coating material to a heated conduit or pipe by means of a continuous belt which conveys a resin layer of uniform thickness to such pipe.

A preferred embodiment of this invention is directed to a means of applying the powder to the heated pipe by spiraling or rolling it and conveying to the pipe a layer of the powdered resin by a tape of heavy paper, a metal belt or conveyor strip made, for example, of reinforced silicone rubber or equivalent high temperature resistant material. The resin can be deposited on the conveyor belt to the same thickness it is desired to be applied on the pipe. This can be accomplished by passing the belt through a slot in a container or hopper filled with the resin so that the belt will emerge therefrom with a layer of resin powder of predetermined thickness. Where powdered epoxy resin is used, we prefer to have a deposit thereof of approximately 10 mils thick. If We use a belt 9 inches wide, working with 4 inch pipe, it would be necessary to adjust the forward movement of the pipe u so that it makes a complete rotation as the pim moves forward approximately 8 /2 inches. It is a simple mechanical matter to coordinate the forward movement and rotation of the pipe with the speed of the belt. The belt or carrier on which the resin is placed can be pressed against the pipe with sufficient force to flatten and spread the excess molten resin that accumulates at the overlaps formed by the spiral wrapping operation. The carrier or belt, as previously indicated, which can be made of steel, preferably stainless steel, returns to the resin hopper and thereafter contacts the pipe, delivering a new supply of the powdered resin in the form of a layer of the desired thickness.

It is possible to use a plastic film carrier such as, for example, polypropylene and actually wind it onto the pipe with the epoxy or equivalent resin powders placed between the polypropylene and the pipes surface. By remaining on the outside, the polypropylene has an additional function of protecting the resin layer from absorbing water which, in time, causes a weakening of the bond between the pipe and the resin layer. In this way, the plastic film carrier serves not only as a means of transferring the resin powder in the form of a layer of uniform thickness of the pipe without the use of air for the coating operation, but at the same time such film remains on the coating as a part of the protective system.

In case where the plastic tape employed has an extremely low tensile strength on contact with the heated pipe to be coated, for example, the tensile strength thereof is so low that the tape tends to break or pull apart where it contacts the hot pipe, this ditficulty can be avoided by supporting said tape on its underside by the use of a heavy kraft paper or equivalent material. This may be conveniently accomplished by interwinding lengths of plastic tape and paper to form rolls that can be dispensed from conventional wrapping equipment, if desired. Actually, the resin coating can be applied in accordance with our invention by spreading the non-tacky, finely divided resin in a substantially uniform layer on a strip of heavy paper and thereafter spirally winding the paper onto the heated pipe. The freshly wrapped pipe, which is still hot, may be cooled in the customary manner, as by spraying water thereon.

The temperature to which the pipe is heated for the coating operation may vary widely. In general, it may be said that such pipe should be heated at least to the melting point of the resin being applied, but below the decomposition temperature thereof. It is preferable to heat the pipe to a temperature such that after a period, for example, of from about 5 seconds to 2 minutes contact of the resin with the pipe, the resin becomes molten and fiowable. Pipe heated to temperatures of the order of 375 F. to about 600 F. will, in general, be found suitable for coating with epoxy, polypropylene, vinyl, and polyethylene resins. Such temperatures assure good coverage of the pipe with a relatively uniform resin layer and minimize the occurrence of voids or holidays therein. In any event, after the layer of applied resin particles has had an opportunity to melt, it becomes solidified and bonds to the pipe surface on cooling.

Referring again to the drawing, a pipe 2 (heated to about 450 F.) rests on supporting wheels 4 carried on shafts 6. Pipe 2 also rests on stainless steel belt 8 running on rollers 10 and powered by motor 12 which turns drive roller 14. On top of pipe 2 are stabilizing wheels 16 which aid in preventing it from vibrating or jumping off wheels 4 during the coating operation. Belt 8 runs through a slot in the lower part of hopper 18 where coating material 20 is spread evenly on belt 8 as it travels toward pipe 2. A spray assembly 22 having a header 24 with 120 opposed jets 26 surround the freshly coated pipe. Affixed to header 24 is a pipe 28 through which water or other suitable coolant flows to quench or cool the molten coating shortly after it has been applied. Beneath the lowermost side of belt 8 'is a group of cold air jets 30 which may be employed to remove heat from belt 8 where relatively low melting point resins are used or conditions are otherwise such that said belt is at or about the melting point of said resin as belt 8 travels through hopper 18.

In operation, pipe 2 is heated to about 450 F. and force-:1 in a right-to-left direction while supported by wheels 4 and held in the place thereon by wheels 16. Pipe 2 also rests with sufficient pressure on belt 8 so that when the latter moves in a counterclockwise direction, pipe 2 is caused to roll in the opposite direction picking up finely divided epoxy resin 20 laid on belt 8 in a layer of substantially uniform thickness, for example 10 mils, when it passes through the slot in the lower part of hopper 18. When the temperature of the pipe is about 450 F., the melting point of the resin employed preferably should be of the order of 250 F. to 275 F., although epoxy resins having melting points of from about F. to about 300 F. may be used. In any event, the pipe should not be heated to a temperature that wiil cause decomposition of the resin.

The speed at which motor 12 drives belt 8 should be regulated to conform to the rate at which pipe 2 travels along wheels 4. By simple trial and error, the proper combination of pipe and belt speeds can be readily determined where resin 20 is applied at a uniform rate from hopper 18 onto belt 8. As the freshly applied resin melts on the hot surface of pipe 2, it forms a tightly adhering coat which is quenched to normal atmospheric or a convenient handling temperature as the coated pipe passes through spray assembly 22. This enables the freshly coated pipe to be removed by hand from the machine and stacked on a rack, if desired.

The procedure, as illustrated in the drawing produces a pipe with a resin coating which in all respects equals that obtainable by the fluid bed or cloud chamber techniques without the above mentioned disadvantages of these methods.

Although epoxy resins in a finely divided form, e.g., -40 to +100 mesh, serve as excellent material for use in the process of our invention, other plastics of similar particle size such as polyethylene, polypropylene and polyvinyl chloride may be likewise employed. In general, it may be said that the particle size of the resins used is not critical; the main requirement being that the particles be sufficiently small so that they melt and flow readily, for example, within 5 to 10 seconds after contact with the heated pipe.

The term carrier belt, as used in the accompanying claims, is intended to refer to the continuous band or belt shown in the drawing.

What we claim is:

1. The method of applying a protective coating to the exterior surface of a conduit which comprises placing a substantially uniform layer of a coating material in nontacky particulate form onto a high temperature resistant continuous carrier belt which is at a temperature below the melting point of said coating material, heating said conduit at least to the melting point of said material but below the decomposition temperature thereof, thereafter turning said conduit at right angles to its longitudinal axis while still hot and bringing said layer while supported by said belt into contact with said turning conduit, moving said conduit in the direction of said axis, whereby said material is applied to said conduit in a layer of contiguous convolutions and simultaneously separating said applied layer from said belt while said belt is in motion, and removing the heat in said belt resulting from said coating operation so that the temperature of said belt is brought below the melting point of said coating material prior to repeating the above cycle.

2. The method of claim 1 wherein said belt is constructed of metal.

3. The method of claim 2 wherein said metal is stainless steel.

5 6 4. The method of claim 1 wherein said belt in con- 2,120,309 6/1938 Carson 117-94 XR structed of reinforced silicone rubber. 2,737,461 3/1956 Heisler et a1. 117-19 XR 2,377,608 6/1945 Bronson 156-487 References Cited 1 UNXTED STATES PATENTS 5 EARL M. BERGERT, Primary Examiner.

1,979,656 11/1934 Whitman. PHILIP DIER,Examin@r- 

